Abstract
Essential trace metals are well known for their environmental toxicity and for being part of complex bio-chemical cycles. Their role as critical micronutrients, delivering vital health benefits, is less widely discussed and understood, holding back strategies for combatting malnutrition. Crops grown on many Indian soils suffer from deficiencies in essential metals, notably iron (Fe), zinc (Zn), and molybdenum (Mo). The list of deficient metals will likely grow due to increasing future crop demand. Geostatistical analysis of soils and farmyard manure (FYM), the predominant fertiliser, implies that residual oxide minerals carry high concentrations of the essential trace metals Fe, Zn, copper (Cu), chromium (Cr), nickel (Ni), cobalt (Co), manganese (Mn) not only in soil but also in FYM (especially Fe, Cr, Cu, Co and Ni). A geochemical survey across a road traverse of 600 km, encompassing an area of c. 15,000 km2, was conducted in Central India to evaluate reported essential metal deficiency in key agricultural topsoils. Importantly, our evaluation of the element cycling in this system reveals that despite high bulk concentrations, some key metals remain bio-unavailable. In effect, the existence of refractory (weathering-resistant) oxides is likely a significant factor for deficiency symptoms in the soil–plant-fertiliser cycle. Further, mass balance calculations of the bioavailable pool of metals imply that only Fe and Mn are present in sufficient quantities to combat deficiency problems. Notwithstanding this limitation of FYM, its high organic carbon content, as well as its importance for Zn, Cu and Fe, validates its traditional use to maintain the fertility and physical condition of Indian topsoils.
Highlights
Essential metals are often referred to as ‘heavy metals’ in the literature and there is ample evidence that high concentrations in soils cause risks to human health, plants, animals and ecosystems
Fertiliser requirements for other essential metals are, to our knowledge, not published. Following this estimate approach using the least “soil/oxide contaminated” farmyard manure (FYM) sample (P18), the metal concentrations determined from this study indicate that the amount of Fe and Mn (155%) is sufficiently high to effectively counterbalance the net output of bio-available essential metals
Chemical and associated data analysis of Central Indian agricultural topsoils and FYM reveals the following: (1) Geochemistry can contribute to food security plans by helping to better quantify the inventory of bioessential minerals from those that “immobilise” key bio-available nutrients in the natural cycling of soil-crop-FYM-fertiliser application
Summary
Essential metals are often referred to as ‘heavy metals’ in the literature and there is ample evidence that high concentrations in soils cause risks to human health, plants, animals and ecosystems. In small quantities, essential metals are indispensable micronutrients for plants, animals and humans [2] The natural availability of essential metals is not ubiquitous or uniform, so that different regions of the world can have deficiencies and/or surfeit of one or more of these elements. These variations are being mapped to understand current and future agricultural potential given predicted future rise of population. Mn deficiency is emerging as a threat to sustaining high levels of food production [6]
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